1. Field of the Invention
This invention relates to electric resistance welded steel tube for machine structural use, more particularly to such tube exhibiting excellent machinability.
2. Description of the Prior Art
Advances in machining technology and increased production of automobiles and other machines have created a rising need for steel materials with outstanding machinability. Tube for machine structural use has been no exception. Automation and other measures implemented to enhance the efficiency of machining processes has led to strong demand for quality structural tube materials exhibiting good chip breakability and machined surface roughness.
For improving the machinability of general steels for machine structural use there have been developed and practically applied a number of resulfurized free-cutting steels, leaded free-cutting steels and calcium-containing free-cutting steels as well as composites of the foregoing. Examples of such steels are disclosed, for example, in Japanese Patent Public Disclosures Sho 55-85658, Sho 57-140853 and Sho 62-33747.
As disclosed in Japanese Patent Publication Sho 61-16337, in some such steels the degradation of mechanical properties caused by the inclusions formed by the addition of such free-cutting elements is suppressed by limiting the amount of Sol.Al and specifying the S, Ca and 0 contents.
However, these conventional free-cutting steels cannot be fabricated into steel tube for machine structural use by electric resistance welding. This is because the inclusions formed by the free-cutting elements in these steels have the general effect of degrading mechanical properties and the specific effect of degrading electric resistance weldability, and the resulting weld cracking and poor performance in ultrasonic testing (UST) of the electric resistance welded steel tube causes a marked decrease in product yield.
These problems cannot be completely overcome even by the teaching of the aforementioned Japanese Patent Publication Sho 61-16337. Moreover it is ordinarily difficult to reduce the O content of Si killed steel to 0.0040% or less.
While an attempt might be made to produce electric resistance welded steel tube by limiting the addition of the free-cutting elements within the range in which there is no large reduction of yield, such an expedient will not enable production of electric resistance welded steel tube with adequate machinability since addition of the free-cutting elements within such a range is not sufficient for preventing the degradation of chip breakability that generally occurs when the tube is subjected to cold drawing or other types of cold processing.
The object of the present invention is to overcome the foregoing problems and provide electric resistance welded steel tube for machine structural use which is able to respond to the need for improved machinability.